properties-of-carbohydrates

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29 Terms

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Carbohydrates
A large group of organic compounds occurring in foods and living tissues, including sugars, starch, and cellulose.
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General formula of carbohydrates
Cn(H2O)n, indicating that they contain hydrogen and oxygen in the same ratio as water (2:1).
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Monosaccharide
The simplest form of carbohydrates, consisting of single sugar units like glucose and fructose.
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Disaccharide
A carbohydrate formed from two monosaccharides, such as sucrose or lactose.
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Polysaccharide
Carbohydrates that are composed of long chains of monosaccharide units, including starch and cellulose.
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Molisch’s Test
A general test for carbohydrates involving concentrated sulfuric acid that produces a purple-colored product.
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Benedict’s Test
A test that identifies reducing sugars based on color change when heated, indicating the presence of free ketone or aldehyde groups.
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Picric Acid Test
A test that uses picric acid to react with reducing sugars, resulting in a red-colored picramic acid.
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Moore’s Test
A test for reducing sugars that changes color from yellow to dark brown when heated with sodium hydroxide.
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Barfoed's Test
A test designed to distinguish between reducing monosaccharides and disaccharides, identifying the faster-reducing monosaccharides.
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Seliwanoff's Test
A test to differentiate aldoses from ketoses based on the speed of dehydration and the color of the product formed.
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Bial's Test
A test to distinguish between pentoses and hexoses by observing color changes when reacting with Bial's reagent.
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Iodine Test for Starch
A test that indicates the presence of starch through the formation of an intense blue-black color with iodine.
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Cellulose

Polysaccharide

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Fructose

Monosaccharide

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Sucrose (table sugar)

Disaccharide

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Glycogen

Polysaccharide

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Glucose

Monosaccharide

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Galactose

Monosaccharide

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Ribose

Monosaccharide

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Lactose (milk sugar)

Disaccharide

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nucleophilic substitution, oxidation-reduction, and rearrangements

These mechanisms in Carbohydrate Chemistry are types of chemical reactions that carbohydrates undergo to form different structures and functions.

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Mutarotation of Monosaccharides

Concept: Monosaccharides exist in equilibrium between their open-chain and cyclic forms. In aqueous solution, they undergo mutarotation, where the α- and β-anomers interconvert via the open-chain aldehyde (or ketone) form.

Mechanism:

  • The hemiacetal/hemiketal bond opens to form the open-chain aldehyde (or ketone) intermediate.

  • The molecule can then cyclize again, leading to a different anomeric form.

  • This process is catalyzed by acid or base.

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Glycosidic Bond Formation (Acetal Formation)

Concept: A glycosidic bond forms when the hydroxyl group of one carbohydrate reacts with the anomeric carbon of another carbohydrate under acid catalysis, leading to the formation of disaccharides or polysaccharides.

Mechanism:

  1. Protonation of the anomeric hydroxyl group increases its leaving group ability.

  2. The departure of water generates a carbocation (or an oxocarbenium ion).

  3. A second carbohydrate nucleophile attacks the anomeric carbon.

  4. Deprotonation results in the formation of a stable glycosidic bond.

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Oxidation of Monosaccharides

Monosaccharides can be oxidized at different positions:

  • Aldonic acids (oxidation at C1) → Example: Glucose to gluconic acid.

  • Uronic acids (oxidation at C6) → Example: Glucose to glucuronic acid.

  • Aldaric acids (oxidation at both C1 and C6) → Example: Glucose to glucaric acid.

Mechanism (Glucose to Gluconic Acid):

  1. The aldehyde form of glucose is oxidized by an oxidizing agent (e.g., Tollens' reagent, Benedict’s reagent).

  2. The aldehyde group is converted into a carboxylic acid.

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Reduction of Monosaccharides (Formation of Sugar Alcohols)

Aldoses and ketoses can be reduced to form sugar alcohols (alditols) by using reducing agents like sodium borohydride (NaBH₄).

Mechanism:

  1. The carbonyl group (aldehyde or ketone) is attacked by hydride (H⁻) from NaBH₄.

  2. This converts the carbonyl to a hydroxyl (-OH) group, forming a polyalcohol.

Example:

  • Glucose → Sorbitol (by reduction of the aldehyde).

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Epimerization (Base-Catalyzed Isomerization)

Concept: A monosaccharide can undergo base-catalyzed enediol rearrangement, leading to epimers (differing at one chiral center).

Mechanism (Glucose to Mannose):

  1. Base deprotonates the α-hydrogen of the aldehyde form of glucose.

  2. The enolate intermediate forms.

  3. Reprotonation at a different position leads to the formation of mannose.

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Benedict’s and Fehling’s Test (Detection of Reducing Sugars)

Reducing sugars (like glucose) react with Cu²⁺ in an alkaline solution to form a red precipitate (Cu₂O).

Mechanism:

  1. The open-chain form of glucose reduces Cu²⁺ to Cu⁺.

  2. Cu⁺ further precipitates as Cu₂O (red color).

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Hydrolysis of Polysaccharides

Concept: Polysaccharides like starch or cellulose can be broken down into monosaccharides by hydrolysis.

Mechanism (Acid-Catalyzed Hydrolysis of Starch):

  1. Acid protonates the glycosidic bond, making it more susceptible to nucleophilic attack.

  2. Water attacks the anomeric carbon.

  3. Cleavage of the glycosidic bond occurs, producing smaller sugar units.